1. Field of the Invention
The present invention relates to a differential limiting device with an electronically controlled multiple-disc clutch which variably provides a differential limiting torque by electronically controlling the magnitude of engaging force of the multiple-disc clutch operably arranged between a differential case and a differential side gear.
2. Description of the Prior Art
Recently, there have been proposed and developed various multiple-disc clutch type electronically controlled differential limiting devices, such as a control device for limited slip differentials. One such multiple-disc clutch type electronically controlled differential limiting device has been disclosed in Japanese Patent First Publication No. 62-103227. The prior-art multiple-disc clutch type electronically controlled differential limiting device includes at least one differential gear arranged between wheel axles for distributing an engine output torque into right and left drive shafts respectively connected to right and left driven wheels while permitting a differential action necessary on turns, a hydraulically actuated multiple-disc clutch operably disposed between a differential case and a differential side gear for properly limiting the differential action, an electronically-controlled hydraulic, unit which hydraulically actuates the clutch to increasingly compensate an engaging force of the clutch in accordance with an increase in the speed difference between the right and left driven wheels. For example, during traveling of the vehicle on a split-.mu. road wherein one of right and left driven wheels lies on a high-frictional road surface such as a dry pavement and the other driven wheel lies on a low-frictional road surface such as a muddy road, wet or icy roads, the wheel-speed difference tends to increase. The above-noted differential action is rather undesirable in view of running stability of the vehicle during traveling of the vehicle on a split-.mu. road. Thus, the conventional differential limiting device is responsive to occurrence of excessive wheel-speed difference to increasingly compensate a clutch engaging force in order to reduce the wheel-speed difference by providing a proper differential limiting torque, and thereby suppresses undesirable differential action. However, there is a tendency judder or shudder of the clutch to occur owing to a clutch friction factor versus wheel-speed difference characteristic, in the event that, during starting of the vehicle on a split-.mu. road, the differential limiting device acts to rapidly increase the magnitude of a controlled hydraulic pressure fed to the multiple-disc clutch so that a differential limiting torque is rapidly increased to rigidly interconnect right and left driven wheels and consequently to suppress excessive wheel-speed difference just after starting on the split-.mu. road. The above-noted clutch friction factor versus wheel-speed difference characteristic will be hereinafter referred to as a ".mu.-v characteristic". As is generally known, the multiple-disc clutch has a first series of clutch plates mounted on the differential case and a second series of clutch plates mounted on the differential side gear. The two series of clutch plates are positioned alternately to form a multiple-disc clutch. In case that the flatness of each clutch plate of the multiple-disc clutch is degraded, the coefficient of friction of the clutch is in general lowered in accordance with an increase in the wheel-speed difference between right and left driven wheels, as appreciated from the .mu.-v characteristic illustrated by the broken line of FIG. 4. Therefore, when the wheel-speed difference v increases during starting of the vehicle on a split-.mu. road, the coefficient .mu. of friction of the clutch is lowered. In response to excessive increase in the wheel-speed difference, the differential limiting device operates to rapidly increase the differential limiting torque, i.e., the clutch engaging force. In this manner, the excessive wheel-speed difference v is effectively suppressed through one cycle of the differential limiting control. After termination of the differential limiting control, the differential limiting torque, i.e., the clutch engaging force is maintained at a low level owing to less wheel-speed difference. Under this condition, there is a tendency for excessive wheel-speed difference to develop again due to the slipping less-traction wheel and the non-slipping greater-traction wheel on the split-.mu. road. In this manner, the wheel-speed difference tends to fluctuate owing to differential limiting controls repeatedly executed during starting of the vehicle on the split-.mu. road. Necessarily, the coefficient .mu. of friction of the clutch also varies according to fluctuations in the wheel-speed difference. As is well known, the differential limiting torque, that is, the clutch engaging force varies in proportion to the product of the hydraulic pressure applied to the clutch and the friction factor .mu. of the clutch. Thus, the fluctuations in the coefficient .mu. of friction of the clutch cause the differential limiting torque to fluctuate. As set forth above, the conventional differential limiting device suffers from the drawback that the vehicle experiences shudder of the multiple-disc clutch employed in the differential limiting device owing to fluctuations in the coefficient .mu. of friction of the clutch, during starting of the vehicle on a split-.mu. road. Such fluctuations in the differential limiting torque act as input vibrations and results in two types of torsional resonance. A first torsional resonance result from a first torsional vibration system which is constructed by a slipping less-traction wheel acting as a mass, and right and left drive shafts cooperatively acting as a torsional spring. An inherent frequency of the first torsional vibration system is determined by two factors, namely a moment of inertia of the mass (slipping less-traction wheel) with respect to the axis of the drive shaft and a spring-rigidity of the respective drive shaft serving as the torsional spring. In the torsional vibration system, the moment of inertia of the mass is also known as a "rotational inertia of the mass". A contact point of a non-slipping greater-traction wheel on the road surface serves as a stationary point of the first vibration system. As appreciated from the above, a relatively long-distance of torsional spring constructed by the two drive shafts is connected in series to the mass constructed by the slipping less-traction wheel. Thus, the first vibration system oscillates at a relatively low resonance frequency such as 7 Hz which test results are experimentally assured by the inventors of the present invention. A second torsional resonance results from a second torsional vibration system which is constructed by a peripheral member of the differential acting as a mass, and a respective drive shaft acting as a torsional spring. In the case that the drive shaft consists of a main drive-shaft portion connected to the corresponding wheel and a flanged drive-shaft inner joint portion splined to a differential side gear, the differential side flange of the inner joint portion has a relatively large rotational inertia. Since the side flange corresponding mainly to the peripheral member of the differential is arranged in the vicinity of the multiple-disc clutch serving as a torsional vibration exciting source, the peripheral member (mass) oscillates at a relatively high resonance frequency such as 150 Hz, as compared with the first torsional vibration system. The former 7 Hz torsional vibration will be hereinbelow referred to as a "7 Hz shudder", while the latter 150 Hz torsional vibration will be hereinbelow referred to as a "150 Hz shudder".
To prevent the above-noted 7 Hz shudder, it is effective to increase rigidities of the respective drive shafts and in addition to reduce a negative gradient of the .mu.-v characteristic of the multiple-disc clutch. The high-rigidity drive shafts and the reduction of the negative gradient of the .mu.-v characteristic may effectively contribute to suppress the 7 Hz shudder.
On the other hand, to solve the 150 Hz shudder problem, it is effective to change the .mu.-v characteristic from minus-gradient tendencies to less-gradient tendencies, that is, to vary the same to a flat .mu.-v characteristic according to which the coefficient .mu. of friction of the clutch is almost constant irrespective of a change in the wheel-speed difference v. In actual, the provision of the flat .mu.-v characteristic means improvement of clutch-plate materials and enhancement of machining accuracy on the clutch plate surface. To obtain an ultimate flatness of the clutch plate surface, the number of polishing processes of the clutch plate must be necessarily increased. This results in an increase in production costs.